[Fluxus] physics commits

[David Griffiths]

Hi all,

Some much needed care and attention for the physics system (on the
racket branch atm):

* Crash fix with joints, which are now deleted when one of the objects
they are attached to is deleted.
* Zero scales in active objects resulted in crashes, this is now fixed.
* Removed some duplicated code in Physics.cpp
* The physics system does a scenegraph traversal each time a primitive
is deleted, making sure all it's children are removed. This means
locators can be used as owners of objects in the physics system.

This makes lazybots better for livecoding - as bots can now be
programmed/built while they are physical, without needing the other bots
to be restarted each time.

The code for lazybots has moved btw, to here: 
http://gitorious.org/flotsam/flotsam/trees/master/lazybotz

cheers,

dave